The only real "solution" to the cross-class skill issue would be to level-restrict cross-class skills. If you just give a high-level WHM all their cross-class skills, that gives them a major advantage over a level-appropriate WHM, which is not good. It doesn't make any sense, but sometimes games have to accept some weirdness due to gameplay mechanics.

As for GC leves, they're solo leves. That's the point. It's no more "inconsistent" than the fact that some MSQ instances are solo (and don't allow use of a chocobo) and others use the duty finder.